Rapid solid-state synthesis of group 6 nitrides, carbon nanotubes and graphite-encapsulated metal nanoparticles.

摘要

The future of technology is dependent on high quality materials. New synthetic methods that are rapid and low-cost routes must be developed to meet new technological demands. The research presented here explores a novel route to high quality materials on rapid time scales called solid state metathesis. The synthetic process involves molecular precursors that react rapidly to form salt by-products which drive the reactions and often reach lead to reaction temperatures in excess of 1300 K. Due to the large amount of heat produced in these reactions, transition metal nitrides such as γ-Mo ₂N and CrN have proven difficult to synthesize. By applying pressures of >40 kbars (>40,000 atmospheres) to solid-state metathesis reactions, these nitrides are now accessible. Cubic γ-Mo₂N and CrN can also be synthesized at ambient pressures by adding ammonium chloride, NH₄ Cl, to solid-state metathesis reactions. Ammonium chloride enables the synthesis of these nitrides by lowering the reaction temperature and serving as an active nitriding source. An advantage to synthesizing γ-Mo ₂N at ambient pressure is the synthesis of high surface area materials, which could potentially be used as hydrodesulfurization and hydrodenitrogenation catalysts. Cubic MoN, the metastable mononitride of molybdenum and nitrogen, has been synthesized in a solid solution with NbN by solid-state metathesis reactions at ambient pressure and in confinement under pseudo-constant volume conditions. The superconducting transition temperature (T_c) for Mo_xNb_1-xN range linearly from 15 K to 11 K as x goes from 0 to 0.25, the apparent stability range for the solid solution. Solid-state metathesis reactions have proven effective at synthesizing nanometer-scale materials. By reacting C₂Cl₆ and Li₂C ₂ in the presence of a 5 mole percent CoCl₂ catalyst, single- and multi-walled carbon nanotubes are synthesized. If a catalyst is not present, only graphite is produced. By increasing the molar concentration of the metal halide catalyst (e.g. CoCl₂, FeCl₃, or NiCl₂), graphite encapsulated metal nanoparticles (e.g. Co, Fe or Ni, respectively) can be synthesized in relatively high yield. The use of long chain carbon halides as reaction precursors is discussed as a means to lower reaction temperature and increase the yield of graphite encapsulated metal nanoparticles.